Ionic liquids (ILs) have been used in lieu of traditional solvents in organic synthesis due to their unique physiochemical properties such as low volatility, significant thermal stability, and ease of recovery. ILs offer clean solvent-free environment to carry out a variety of eco-friendly reactions.
Imidazole or pyridine-based ionic liquids with varied range of counter anions have commonly been used as catalysts in synthetic chemistry. However, the inert nature of imidazole and well-known toxicity of pyridine is an impediment towards further expanding the scope of ionic liquids. Herein, we have employed non-toxic quinoline as a head group for the synthesis of N-alkylated ionic liquids.
Quinoline is an important scaffold in a number of compounds of medicinal and pharmaceutical interest. Quinoline-based ionic salts have not yet been explored as catalysts in organic synthesis. Quinoline-based ionic liquids can be easily prepared via a simple alkylation and subsequent counter-ion exchange under conventional environment, without employing any strict controlled reaction conditions. In this study, the counter anions (i.e. I−, Br−) of the corresponding ionic salts were exchanged with fluoride ions (F−), which is known to act as a mild base in various organic reactions.
The quinoline-based ionic liquids with fluoride counter anion have a close resemblance with tetrabutyl ammonium fluoride (TBAF), a well-known catalyst in organic reactions. In order to explore the catalytic potential of synthesized QuFs, click reaction, and other multicomponent reactions were carried out under solvent-free conditions.
Click chemistry was used for the synthesis of 5-(p-methylphenyl)-1H-tetrazole (7). Keeping in view the hygroscopic nature of ionic liquids, the effect of successively increasing concentration of water on synthetic output of the reaction employing QuF (4) as ionic liquid was investigated in detail (FIG. 1).
Tetrazole serves as an isosteric substituent for carboxylic acid group in many compounds of pharmaceutical interest. The tetrazole moiety improves the metabolic resistance and also the pharmacokinetic properties of drugs. Multicomponent reactions carry unique place in synthesis and medicinal chemistry to build different compounds libraries for biological screening purposes.
A wide range of applications of ionic liquids have been reported owing to a number of unique physical properties including thermodynamic and ion transport properties. These properties of ionic liquids vary widely depending on the nature of the cation and anion, therefore, such properties should ideally be known prior to the use of an ionic liquid.
Transport properties of ions are of great significance that need to be estimated for the design of new ionic liquids for use as electrolytes or in electrochemical devices. Different properties of ionic liquids can be assessed by means of experimental techniques and can also be simulated by means of molecular dynamics (MD) simulation. Hence structural and dynamic properties of QuF derivatives in bulk, including self-diffusion coefficients were investigated by employing molecular dynamics simulations.
Density functional theory (DFT) calculations were used to gain structural insight into QuF ionic liquids. The stability of such ionic liquids is of paramount interest, hence binding energy calculations were performed in comparison to tetrabutyl ammonium fluoride (TBAF), a commercially available ionic liquid.